Mixed micelle formation of anionic and nonionic surfactants

Mixed micelle formation of anionic and nonionic surfactants

Mixed Micelle Formation of Anionic and Nonionic Surfactants F U M I K A T S U T O K I W A ANI) N O B O R U 3,iORIYAMA Research Laboralories, Kao Soap...

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Mixed Micelle Formation of Anionic and Nonionic Surfactants F U M I K A T S U T O K I W A ANI) N O B O R U 3,iORIYAMA

Research Laboralories, Kao Soap Company, Wakayama-shi, Japan Received December 30, 1968; revised February 17, 1969 The effects of a series of nonionic surfactants, dodecyl polyoxyethylene ethers (DPE), on the micellization of an anionic surfactant, sodium dodecyl sulfate (NaDS), have been studied as a function of the mole ratio of NaDS/DPE and of the polyoxyethylene chain length of the DPE molecule. Attention has been paid to changes in the degree of ionic dissociation of NaDS brought about by its incorporation in mixed micelles. The degree of ionic dissociation of the mixed micellar species was evaluated both from osmotic and pNa data. It has been shown that the degree of ionic dissociation of the ionic surface-active material in the mixed micelle increases as the proportion of the nonionic material increases and as the polyoxyethylene chain of the nonionic material is lengthened. The specific conductivities of the mixed solutions of NaDS and DPE at low concentrations are smaller than those of the solutions of NaDS alone, whereas at high concentrations the former are greater than the latter; the results would be explained by the degree of ionic dissociation and the mobility of the mixed micelles.

INTRODUCTION

understand the mixed micelles composed of ionic and nonionic surfactants. Particular attention has been paid to changes in the degree of dissociation of the ionic surfactant brought about b y its incorporation in mixed micelles. I n addition, an electrical conductivity study of the mixed solutions of N a D S and D P E has also been included in this work. EXPERIMENTAL

Above the critical micelle concentration (CMC) an ionic surfactant of the type sodium dodecyl sulfate (NaDS) aggregates to form micelles, the charge of which depends on the proportion of the counter ions which are bound to the micelle surface. The activity of the monomerie surface-active species can be considered to remain constant above the CN[C. The micellization of a nonionic surfacrant of the type dodecyl polyoxyethylene ether (DPE) involves a similar aggregation although the charge effect is absent. From light scattering (i, 2), electrophoresis (3, 4), conductivity (5, 6), vapor-pressure depression (6), and other types of measurements, we are able to deduce that ionic surfactants interact with nonionic surfactants to form weakly charged mixed micelles. In the present work, the effect of DPE on the micellization of NaDS has been studied by changing the mole ratio of NaDS/DPE and the chain length of the polyoxyethylene part in the DPE molecule in order to better

Materials. Sodium dodecyl sulfate (NaDS), C12H25OSO3Na, was prepared from dodecyl alcohol of a high purity by sulfation with ehlorosulfonie acid according to the method of Dreger et al. (7). The sample was purified by repeated reerystallization from ethyl alcohol, followed b y extraction with petroleum ether. Dodeeyl polyoxyethylene ethers ( D P E ) , C12H250(CH2CH20)nH with different values of n, were prepared from the same dodecyl alcohol b y addition of ethylene oxide, with sodium hydroxide used as a catalyst. Polyethylene glycol, a by-product of the reaction, was removed by the solvent-

Journal of Colloid and Interface ~cience, Vol. 30, No. 3, J u l y 1969

338

MIXED MICELLE FORMATION OF ANIONIC AND NONIONIC SURFACTANTS extraction method (8), using 1-butanol-saturated water and water-saturated 1-butanol. Paper chromatography showed the purified samples to be free of polyethylene glycol except for the D P E with n of 30, which contained a very small amount of polyethylene glycol. The average numbers of oxyethylene units per molecule of the purified samples, calculated from their hydroxyl values, were approximately 5, 9, 15, and 30; they are designated as DPE-5, DPE-9, DPE-15, and DPE-30, respectively. Vapor Pressure Measurements. T h e v a p o r pressure depression of aqueous solutions oI' NaDS alone, and mixed with D P E , was measured with an osmometer, Hitaehi-Perkin Elmer 5.Iodel 115, at 40°C. This instrument uses thermistors to detect the temperature difference between a drop of solution and a drop of solvent in an atmosphere saturated with solvent vapor; the temperature difference is proportional to the vapor-pressure depression. The instrument was calibrated with sucrose and salt solutions. pNa Measurements. The p N a values of solutions of single and mixed surfaetants were measured at 25 4- 0.1°C with a p N a meter, Hitaehi-ttoriba Model N-5, using a cell of the type Na-glass eleetrode

sample NH4NOa KC1 solution (1.0M) (3.3 M)

calomel electrode

The pNa values were determined by diluting stepwise a concentrated solution of the surfaetant(s) with water, and the values thus obtained were confirmed by making a few measurements on individually prepared solutions. The standardization of the p N a meter used was performed by using standard 1.0 and 0.01 M solutions of sodium chloride. Conductivity Measurements. The conductix4ties of aqueous solutions of single and mixed surfactants were determined using a sealed cell and a conductance bridge, TSa Denpa Model C~\,i-IDB, at 25.0 4- 0.01°C. The water used in preparing the solutions had a specific conductance of 1.0 ~-~ 1.5

- - - I

I

I

t

~--

339 l

~0.02

//

o z "5

.~

_

0.01

E

a
0.02

0.05

Concentration of NoDS (c),mol./I,

Fro. 1. The apparent concentration 5 from the osmotic data against the stoichiometric concentration c for NaDS alone and mixed with DPE-9 at 40°C. Mole ratio NaDS/DPE-9: O, 100/0 (NaDS alone); O, 80/20; ~, 50/50; ~ , 20/80. X 10-6 ohm -1 cm -1. The variation of conductivity with concentration was determined by progressive dilution of a concentrated solution, and the values thus obtained were confirmed by making a few measurements on individually prepared solutions. The observed values were checked by measuring the specific conductivities of standard potassium chloride solutions. RESULTS The results of vapor-pressure measurements are shown in Figs. 1 and 2, where the apparent concentration ~, calculated from the observed vapor pressure depression on the assumption that the material dissociates to a single molecular species and behaves ideally, is plotted as a function of the stoichiometric concentration c of NaDS. Figure 1 gives the plots of ~ vs. c for the mixtures of NaDS and DPE-9 at different mixing ratios, and Fig. 2 gives the plots for the mixtures of NaDS and D P E with different numbers (n) of oxyethylene units per molecule at a constant mixing ratio of 50//50. With NaDS alone, below the C M C t h e slope of the e vs. c curve is coincident with that obtained for a 1-1 type strong electrolyte, but above the C2\IC this slope is drastically reduced. The vapor-pressure deJournal of Colloid and Interface Science, Vol. 30, No. 3, J u I y 1969

340

TOKIWA AND MORIYAMA I

I

I

1

I

3.ok

I

i

t

i

a.5~1~

~0.02

!/ g

~=

~

i

0.01

I

CL

g

c~ <[

{

I I I I 0.01 0.02 Concentration of NoDS (c), mol./I.

I 0 05 I

FIG. 2. The apparent concentration ~ from the osmotic data against the stoichiometrie concentration c for NaDS alone and mixed with DPE with different numbers (n) of oxyethylene units at a constant mole ratio of 50/50 at 40°C. ©, NaDS alone; mixed with DPE with n of 5 (O), n of 9 ((}), n of 15 (~), and n of 30 (®).

3;0

I

l

I

l

.%

2.5

'
!

o_ 1.5

1.0 ____ I i I 1 -2.7 -25 -2.0 -1.7 Ioq concentration of NoDS (mol./I.)

-I,5

FIG. 3. pNa against log concentration for NaDS alone and mixed with DPE-9 at 25°C. Mole ratio NaDS/DPE-9 : ©, 100/0 (NaDS alone) ; O, 80/20; (}, 50/50; ~ , 20/80. (The scale shown is for curve 1; curves 8, 3, and 4 have each been displaced downwards by 0.2, 0.4, and 0.6 pNa unit, respectively.) pressions of aqueous solutions of D P E alone are very small, especially in the cases of D P E - 5 and DPE-9. With the mixtures of N a D S and DPE-9, however, the slope of the curve above the critical concentration increases with decreasing mole ratio of N a D S / DPE-9, i.e., with increasing amounts of the Journal of Colloid and Interface Science, V o l . 30, N o . 3, J u l y 1969

I

-2.7 -2.5 -20 -[.7 log concenlration of NoDS (mol. I.)

-1.5

FIG. 4. pNa against log concentration for NaDS alone and mixed with DPE with different numbers (n) of oxyethylene units at a constant mole ratio of 50/50 at 25°C. ©, NaDS alone; mixed with

DPE withn of 5 (0), n of 9 (~), n of 15 ( ~ ) , and n of 30 (®). (The scale shown is for curve 1; curves 2, 8, 4, and 5 have each been displaced downwards by 0.2, 0.4, 0.6, and 0.8 pNa unit, respectively.) nonionie materials, as seen in Fig. 1. With the mixtures of N a D S and D P E with different values of n at a constant mixing ratio, on the other hand, the slope increases with increasing n, as seen in Fig. 2. I n Fig. 3, the p N a values of aqueous solutions of NaDS alone and mixed with D P E - 9 are plotted against the logarithm of the concentration of NaDS. Figure 4 shows similar plots for the mixed solutions of N a D S and D P E with different n at a constant mole ratio of 50/50. I n each curve, a break point appears at a certain critical concentration; with N a D S alone, the break point corresponds to its C M C (9). The critical concentration at which a break point appears is lowered with decreasing mole ratio of N a D S / DPE-9, and it also decreases with increasing polyoxyethylene chain length of D P E . However, this tendency becomes obscure at low mole ratios of N a D S / D P E and in the case of D P E having a longer polyoxyethylene chain. Above the critical concentration, the slope of the curve increases as the mole ratio of N a D S / D P E decreases or as the polyoxyethylene chain of D P E is lengthened.

MIXED

MICELLE

FORMATION

OF

ANIONIC

/O

5 5

~5



1

O.OI

0.02

0.03

Concentrot on of NaDS (c), m o L / l .

FIG. 5. Specific c o n d u c t i v i t y against concent r a t i o n for N a D S alone and mixed w i t h D P E - 9 at 25°C. Mole ratio N a D S / D P E - 9 : O, 100/0 (NaDS alone); 0 , 8 0 / 2 0 ; 0 , 50/50; ~ , 20/80.

The specific conductivity vs. concentration curves for aqueous solutions of NaDS alone and mixed with DPE areshown inFigs. 5 and 6. As seen in Fig. 5, the sharp break in the curve of conductivity vs. concentration observed for NaDS alone becomes less pronounced as the mole ratio of NaDS/DPE-9 decreases until at low values only a smooth curve is obtained. At low concentrations the specific eonductivities of the mixed solutions are lower than that of the NaDS solutions, but at high concentrations the former becomes higher than the latter. On the other hand, as seen in Fig. 6, the conductivity at a constant mixing ratio at a given concentration in the region of high concentrations decreases with increasing number of n of DPE, except for the mixture of NaDS/DPE-5. The values of conductivity for this mixture lie between the values for DPE-15 and DPE-30.

AND

NONIONIC

SURFACTANTS

341

ionic micelles. At low concentrations, the DS- ions are incorporated into the nonionie micelles of DPE, which are formed at a much lower concentration than those of the ionic NaDS (6). With increasing concentration, the content of NaDS in the mieelles increases, and finally mixed mieelles of constant composition Mll be formed. With respect to counterion binding, the degree a of dissociation of NaDS in the mixed micelle species can be calculated both from the vapor pressure and the pNa data. Although the general trend of dissociation with micelle composition is the same for both methods, i.e., the degree of dissociation increases with increasing nonionic content and with increasing polyoxyethylene chain length of DPE, the absolute magnitudes of the dissociation derived from these two sets of data do not necessarily agree. The assumptions involved in deriving the degrees of dissociation by the two methods have therefore been considered. From Osmotic Data. Now, let us assume the micelles to be polydispersed with the concentration of the monomer species above the CMC remaining constant. If the ith mieelle species, containing ni ionic surfactant molecules and carrying a charge of p~, has a concentration of c~ (in monomer concentral

]

I

I-

I

I

× m o

u

DISCUSSION

The Degree of Ionic Dissociation of Mixed MicelIes. Mixed systems of anionic and nonionic surfaetants are considered to form mixed micelles composed of these two surfactants. The decrease in apparent coneentration of NaDS in the presence of DPE, as seen in Figs. 1 and 2, indicates a gradual uptake of the surface-active ions by the non-

cO

0.01

0.02

Concentration of NaDS

(c),

0.03 mol./I.

Fin. 6. Specific c o n d u c t i v i t y against concent r a t i o n for N a D S alone and mixed w i t h D P E h a v ing different n u m b e r s (n) of oxyethylene u n i t s a t a c o n s t a n t mole ratio of 50/50 at 25°C. O, N a D S alone; mixed w i t h D P E w i t h n of 5 ( 0 ) , n of 9 ( ~ ) , n of 15 ( ~ ) , and n of 30 (®). Journal of Colloid and Interface Science,

Vol. 30, No.

3, J u l y

1969

342

TOKIWA

AND MORIYAMA

tion unit), then the osmotically effective concentration 5 (apparent concentration) of the solution above the CMC may be given by

TABLE THE

DEGREE IN MIXED

I

OF DISSOCIATION SYSTEMS OF NADS

c~ OF MICELLES AND DPE-9

Values of _'vIoleratio of NaDS/DPE-9

where Co is the concentration of fl'ee ionic surfaetant. (The concentration of free nonionic surfactant would be negligibly small. ) T h e quantity of 8c is written b y

8~ = 6 ~

(p~/n~)c~ + ~

(1/ni)ci.

[2]

On the other hand, the stoichiometric concentration c of the ionic surfaetant will be given by c = co +

100/0 80/20 50/50 20/80

[3]

From osmotic data at 4O°C

From pNa data at 25°C

0.16 0.25 0.40 0.65

0.18 0.30 0.47 0.66

TABLE II THE

DEGREE

IN

OF DISSOCIATION

MIXED

WITH

SYSTEMS

DIFFERENT

OF

eL OF

MICELLES

AND DPE

NADS

I:)OLYOXYETHYLENE

CHAIN

LENGTHS AT A M I X I N G M O L E RATIO OF 5 0 / 5 0

and Values of c~

st =

: E c,:.

[4]

Number of oxyethylene units per DPE molecule

From Eqs. [2] and [4],

dc

5 9 15 30

d ~ ci

and neglecting changes in p~ and n~ by dc, we obtain I

d5 _ ~ (p,/ni) dci + ~ ( I / n , ) dc~ dc ~ , dci

[51

If the mieelle distribution is invariant with concentration, then dci/ci = k, a constant independent of the mieelle type, and so

dc

~ c~

Here, if two averages (1/n) and (p/n) are defined as

(1/n} = F_, (1/~)c~/~2 c~, and (p/~} = ~

( P , / ' ~ ) ~ / E ~, ,

then Eq. [6] becomes

d~/dc = (pin) + (i/n) ~ (p/n).

[7]

Therefore, the average degree of dissociation 1 If the micelles are assumed to be monodispersed, the values of p~ and n; are constant and Eq. [5] becomes

d-c/dc = (p/n) + (l/n) ~ pin. Journal of Colloid and Interface Science, Vol. 30, No. 3, July 1969

(NaDS

alone)

From osmotic data at 40°C

From pNa data at 25°C

0.30 0.40 0.51 0.63 (0.16)

0.36 0.47 0.58 0.64 (0.18)

of the micelle, a = (p/n}, can be evaluated from the slope of the e vs. c curves shown in Figs. 1 and 2. The values of a, calculated from the slope of the linear portion of the e vs. c curves as a function of the composition of the mixture and the polyoxyethylene chain length of D P E , are summarized in Tables I and II, from which it is seen that a increases as the ratio of N a D S / D P E decreases and as the polyoxyethylene chain length of D P E increases. The value of a for NaDS, 0.16, may be compared with that for sodium dodecyl sulfonate, 0.14, obtained from osmotic data by Corkill et al. (6). From pNa Data. An evaluation of the binding of counterions by micelles has been made by Botr6 et al. (10). Their treatment assumes, in agreement with generally accepted views, that above the CMC further addition of surfactant merely increases the number of mice]les; their size and the concen-

MIXED MICELLE FORMATION OF ANIONIC AND NONIONIC SURFACTANTS

oo2L

Concentration of NoDS

(c),

mol,/I.

FIG. 7. The activity of counterions (No+) obtained from the pNa data against the concentration of NoDS alone and mixed with DPE-9 at 25°C. Mole ratio NaDS/DPE-9: O, 100/0 (NODS alone); O, 80/20; ~, 50/50; ~ , 20/80. tration of unmicellized species remain constant. Under these conditions, the experimental activity a of eounterions can be expressed by the following equation by formal separation of the contribution of the unmieellized and mieellized molecules, a =

'YexpC =

V-l-, {C0 Jr- ~ ( C

- - C0)}

[S]

where 3'e_~1,is the experimental value of the activity coefficient and ~/+ is the activity coefficient of counterions. Differentiating Eq. [8] ~dth respect to c, under the assumption that % is constant above the critical concentration co, we obtain (Oa/Oc)~+ = ~ + { ~ + (c -

co)(~/dc)}. [9]

Figure 7 shows the curves of the activity of Na + ions vs. the concentration of NoDS which are obtained by replotting the p N a vs. log c curves shown in Fig. 3. As seen in this figure, in the region of concentrations higher than 1.5 X 10-2 (moles/liter), the slope of the curves is almost constant, indicating do~de = O. The a vs. c curves for the systems shown in Fig. 4 are also linear in the region of high concentrations. Then Eq. [9] reduces to do~de = -y+a [10] The value of a can thus be obtained from the slope of the linear portion of the a vs. c

343

curve by assuming 3'+ to be equal to the experimental value at the concentration co where a break point appears in the p N a vs. log c curve. With NoDS alone, the value of Co corresponds to its CMC. With the mixed systems of NoDS and D P E , the value of cois considered to mark the onset of the formation of mixed mice]les of a constant composition. Tables I and II give the values of for NoDS and the mixtures with different N a D S / D P E ratios and different n. (The value of a for NoDS is comparable with the values reported by Phillips et al. (11), Botr~ et al. (10), and Shedlovsky et aI. (12) which are 0.18, 0.16, and 0.22, respectively.) The tendency of the dissociation is similar to t h a t obtained from osmotic data, i.e., the value of a increases as the ratio of N a D S / D P E decreases and as the polyoxyethylene chain length of D P E increases. (The discrepancy in the dissociation values obtained by the two methods might well be partly due to the difference in the temperature of measurement.) I t is important to note that, with respect to the effect of D P E on the degree of dissociation of the mixed micelle, making the m01e ratio of N a D S / D P E decrease at a constant polyoxyethylene chain length corresponds to making the polyoxyethylene chain of D P E lengthen at, a constant ratio of NaDS/DPE. The Electrical Conductivity of Mixed Surfactant Solutions. In the region of low concentrations, the specific conductivities K of the mixed solutions of NoDS and D P E are smaller than those of the solutions of NoDS alone, as seen in Figs. 5 and 6. This results from the incorporation of the ionic component (DS- ions) into the nonionic micelles. In the region of high concentrations, on the other hand, the values of ~ for the mixed solutions are greater than those for the NoDS solutions. In the systems given in Fig. 5, the effect of the mole ratio of N a D S / D P E - 9 on the conductivity is similar in tendency to the effect on the degree of dissociation of the mixed micelles. The increase in conductivity Journal of Colloid and Interface Science, Vol. 30, No. 3, July 1969

344

TOKIWA AND MORIYAMA

at high concentrations may therefore be ex- the above consideration. This is supported plained by the increase in dissociation of the by the results obtained in mixed solutions of mixed micelles with decreasing mole ratio of NaDS and polyethylene glycol, which will NaDS/DPE-9. However, in the systems of be reported in a forthcoming paper. NaDS and DPE with various lengths of ACKNOWLEDGMENTS polyoxyethylene chain, the trend of the inThe authors express their thanks to Dr. H. crease in conductivity does not agree with Kita, Director of the Research Laboratories, for the increase in dissociation of the micelles; his encouragement and permission to publish this namely, the conductivity increases in the paper. order of DPE-9 > DPE-15 > DPE-5 > REFERENCES DPE-30. 1. CORKILL, ,1. M., GOODMAN, J. F., AND 0TTEThe specific conductivities of surfactant WILL, R. H., Trans. Faraday Soc. 57, 1627 solutions above the CMC are considered to (196]). be governed by two factors: the mobility 2. KURIYAMA, K., INOUE, H., AND ~AKAGAWA, of the micelles and the mobility of the free T., Kolloid-Z. u. Z. Polymere 183, 68 (1962). 3. TOKIWA, F., ]. Colloid and Interface Sci. 9.8, Na + ions (in other words, the degree of 145 (1968). dissociation of the micelles). The mobilities 4. NAKAGAWA,W., AND INOUE, I-I., Nippon Kagaof the mixed micelles formed in the soluku Zasshi 78, 636 (1957); Chem. Abstr. 52, tions of NaDS and DPE with relatively 2429h (1958). long polyoxyethylene chains, for example, 5. YODA, O., MEGURO, K., KONDO, W., AND INO, K., Nippon Kagaku Zasshi 77, 905 (1956); DPE-15 and DPE-30, would be smaller than Chem. Abstr. 51, 7805g (1957). those of the micelles formed in the solutions 6. CORIZILL, J. M., GOODMAN, J. F., AND TATE, of NaDS and DPE with shorter chains, J. R., Trans. Faraday Soc., 60, 986 (1964). probably because of the increased affinity 7. DREGER, E. E., KEIM, I., MILES, G. D., of the polyoxyethylene part for water with SHEDLOVSI(Y, L., AND ROSS, J., Ind. Eng. Chem., 36, 610 (1944). increasing number of oxyethylene units per 8. NAOASE, K., AND SAKAOUCm, K., Kokyo molecule. The reduction of the mobility of Kagaku Zasshi 64, 635 (1961); Chem. Abstr. the micelle with increasing polyoxyethyelene 57, 3580f (1962). chain length would affect the specific con9. MYSELS, K. J., AND OTTER, R. J., J'. Colloid ductivity more than the increment of ionic Sci. 16, 462 (1961). dissociation of the micelle with increas- 10. BOTI~]~, C., CRESCENZI, V. L., AND MELE, A., J. Phys. Chem. 63, 650 (1959). ing polyoxyethylene chain length. Thus, 11. PHILLIPS, J. N., AND MYSELS, g . J., J. Phys. the conductivity behavior of the solutions Chem. 59, 325 (1955). of NaDS and DPE with different polyoxy- 12. SHEDLOVSKY,L., JAKOB, C. W., AND EPSTEIN, ethylene chain lengths could be explained by M. B., J. Phys. Chem. 67, 2075 (1963).

Iournal of Colloidand Interface Science, VoL 30, No. 3, July 1969